The present invention relates to a device having an AF (automatic focusing) capability (function) such as an AF camera. More particularly, the invention relates to a drive method for such a camera adapted to move a movable focusing lens to the focused position.
Recently, various interchangeable lens cameras having an AF capability have been proposed and marketed.
FIG. 1 is a block diagram showing the general arrangement of such a camera. FIG. 1 primarily shows components related to an AF mechanism; that is, some components generally provided for the camera and lens systems are not shown in FIG. 1 for clarity of illustration.
In FIG. 1, reference numeral 11 designates a camera body, and 31, a photographing lens which is detachably mounted on the camera body 11. The camera body and the photographing lens are mechanically connected to each other through a clutch 13 on the body side and a clutch 33 on the lens side, and electrically connected to each other through an electrical contact group 15 on the body side and an electrical contact group 35 on the lens side.
The photographing lens 31 includes a lens system 39 including a lens 37 movable along the optical axis to focus on a designated object, a drive force transmitting mechanism 41 for transmitting the drive force of a drive source (described below) in the camera body to move the movable lens 37 to the focused position, and a lens ROM 43 for storing aperture data for the photographing lens and position data for the movable lens 37.
The components of the camera body 11 include an image pickup section 17 employing, for instance, a CCD (charge-coupled device) sensor receiving a part of the light from the object which has passed through the photographing lens 31. The camera body 11 further includes a control section 19 having a variety of functions such as calculating the amount of defocusing indicating the amount of deviation from the focused position according to an output signal from the image pickup section 17 and determining the correction direction for focusing the lens on the object. The camera body further includes a drive mechanism 25 composed, for instance, of an electric motor 21 for driving the movable lens 37 in the photographing lens 31 and an encoder 23 for detecting the number of revolution of the motor 21. The drive force of the drive mechanism 25 is transmitted through the clutch mechanisms 13 and 33 and the drive force transmitting mechanism 41 to the movable lens 37 to move the latter.
The amount of drive for moving the movable lens 37 to the focused position can be determined from the number of pulses P counted by the encoder 23 detecting the number of revolutions of the motor 21 (hereinafter referred to as a pulse count value P when applicable). In a conventional AF camera, the amount D of defocusing is obtained by the control section 19, and the pulse count value P corresponding to the amount D of defocusing is obtained according to the following equation: EQU P=K.multidot.D (1)
where K is the lens movement conversion coefficient. The coefficient K is determined in advance in such a manner that a pulse count value P which permits the movable lens 37 to be moved, according to the amount of D of defocusing, to the position where the lens is positively focused on the object is obtained.
Each photographing lens has its own coefficient K. The coefficient K is stored in the lens ROM 43 of the photographing lens in advance. In the case where the photographing lens is a zoom lens, a plurality of coefficients are stored in the lens ROM.
The movable lens 37 can be moved to the focused position by continuously applying a constant current to the motor 21, termed a DC control mode or by intermittently applying a current to the motor, termed a PWM (Pulse Width Modulation) control mode. In each of these modes, the movable lens is moved until the number of pulses counted by the encoder 23 reaches the pulse count value P obtained from equation (1).
Driving the movable lens in the DC control mode or in the PWM control mode under predetermined driving conditions though suffers from the following difficulties:
When the ambient temperature changes, the viscosity of grease in the drive mechanism or in the drive force transmitting mechanism for driving the movable lens changes. This changes the load on the lens driving motor. Accordingly, when the lens is driven under the present conditions, the lens moving speed changes with the ambient temperature.
Furthermore, the lens moving speed changes depending on the posture of the camera due to the effects of gravity; that is, the gravitational force acting on the focusing lens in different when the focusing lens is moved in the direction of gravity and in the opposite direction under the same driving conditions. Specifically, when the focusing lens is moved in the direction opposing the force of gravity, the lens driving load on the motor is increased. As a result, the lens moving speed in the direction of gravity is different from that in the opposite direction.
The above-described lens moving speed variations prevent quick and accurate achievement of automatic focusing.